Signaling system



April 18, 1939. E VROOM 2,154,921

SIGNALING SYSTEM Filed Dec. 19, 1936 BV/QWM ATTORNEY Patented Apr. 18, 1939 UNITED STATES PATENT OFFICE Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application December 19, 1936, Serial No. 116,696

4 Claims.

'Ihe invention relates to signaling systems and in particular to systems employing impulses transmitted in accordance with a code, as in telegraphy and in the dialing of impulses for use in remote switching or other control operations. More particularly, the invention relates to means for insuring against loss of signals due to fading in radio transmission and other interfering effects, particularly those that are selective as to frequency.

It has been proposed to send waves of two frequencies alternately, one to form a marking signal and the other to form a spacing signal, as it has been found that a definite signal is better than a space devoid of a signal, when protection against interference is desired. It has also been proposed to send out waves of two or more frequencies simultaneously to form a signal, because experience has shown that the probability that two or more waves of different frequency will fade out at the same time is much less than the probability that a single wave will fade out at any given time.

In accordance with the present invention two or more waves of different frequency are used to form a signal and it is so arranged that at the receiving station the waves of different frequency are shifted or translated to a common frequency before they are filtered to separate them from extraneous waves or disturbances. The filtered waves, representing one or more of the transmitted, signal-bearing waves, are then used to actuate the receiving relay or other useful device. 'I'he advantages of this system include a simplification of apparatus, particularly a decrease in the number and complexity of the filters required. Other advantages and features will be apparent from the following detailed description taken in conjunction with the accompanying figures, of 40 which Fig. 1 represents in diagrammatic fashion the preferred form for complete transmitting and receiving stations arranged for one-way operation in accordance with the invention;

Fig. 2 shows a modified form of transmitting circuit; and

Fig. 3 shows a system illustrating a modified form of the invention.

Referring to Fig. 1, a pair of tone generators I and 2 supply 2295-cycle-per-second and 2125- cycle-per-second tone currents respectively. A key 3 has a back contact connected to the 2295- cycle generator and a main contact connected to the 2125-cycle generator. The key lever is connected to a line 4 which has branches connected respectively to an inverter 5 and an amplifier 6. Also connected to the inverter is a carrier generator for supplying a 300D-cycle current. The output circuits of the inverter 5 and of the amplifier 6 are jointly connected to a radio 5 transmitter 8 and thelatter is connected to a transmitting antenna 9.

At a receiving station a receiving antenna I0 is provided together with a radio receiver II. The output circuit of the radio receiver is connected to an inverter I2 and an amplier I3. In conjunction with the inverter I2 there is provided a second carrier generator I4. The output circuits of the inverter I2 and of the amplifier I3 are jointly connected to a pair of filters I5, I6, tuned to 2295 cycles an-d 2125 cycles respectively. Connected to the respective outputs of the filters are two rectiers Il, I8 which are in turn jointly connected to a differential relay I9. The relay is coupled to a local circuit or line section 20.

In the operation of the system of Fig. 1, the key 3 is manipulated to form marking and spacing signals alternately in accordance with a code. During a marking signal the key is depressed, thereby connecting the 2125-cycle generator 2 to the line 4. During a spacing signal the key is raised, thereby connecting the 2295-cycle generator I to the line 4. Whichever tone generator is connected at a given instant, let the tone frequency be designated as f. The function of the inverter 5 and carrier generator 'I is to invert the frequency f with respect to 3000 cycles. By this is meant that the inverter upon receiving a tone current of the frequency f, produces in its output circuit a current of the frequency 300D-f. The process of inversion and apparatus for effecting the result are not a part of the present invention and are well known in the art. For an example of an inverter of this sort, reference may be had to U.'S. Patent 1,603,287, issued October 19, 1926 to J. Mills. The frequency in the output of the amplifier 6 is, of course, unchanged and equal to f.

The inverted and uninverted currents from the inverter 5 and the amplifier 6 respectively are impresse-d jointly upon the input circuit of the radio transmitter 8 wherein is produced in Wellknown manner a radio frequency carrier wave modulated by the two-tone frequencies, f and 300D-f. If the carrier frequency is designated by F, then the side frequencies produced are F+f and Fi(3000-f) and either the` upper or the lower or both pairs of side frequencies, with or without the carrier, according to the method of operation desired, are transmitted to and radi- 55 I9 through the action of the rectifier I8.

ated from the antenna 9. The essential requirement of the transmission is that provision be made for transmitting waves from which the tone frequencies f and 3000---1c may be reproduced at the receiving station.

At the receiving location the waves radiated by antenna 9 are intercepted by antenna I0 and detected in the radio receiver II in the usual manner to reproduce the tone frequencies and 3000- i, providing that one or both of the corresponding radio waves are not lost due to fading of the waves in the course of transmission through space between the antennae. Assuming that both tone frequency currents are present in the output of the radio receiver, they are then impressed jointly upon the inverter I2 and the amplifier I3. The inverter I2 is supplied by the source I4 with substantially the same frequency, here 3000 cycles, as was supplied by the source 'l to the inverter 5. The inverter I2 converts the frequency f to the frequency 3000-f, and likewise converts the frequency 3000-f to 3000-(3000--f), or f again. The purpose of this inversion will appear more fully below when the effects of fading are considered. The amplifier I3 passes the frequencies f and 3000- f without inversion.

The inverted and uninverted tone currents are next impressed upon the filters I5 and I6. Filter I5 is designed to pass only frequencies in the neighborhood of the frequency generated by source I, in this case 2295 cycles. Filter I6 is designed to pass only frequencies close to that of source 2, here 2125 cycles. As the frequency f represents either the frequency of source I or the frequency of source 2, it will be noted that the current of frequency f is passed by one of the filters and stopped by the other. 'I'he current of frequency 3000-f is stopped by both filters. In the case of a marking signal, f is 2125 cycles as illustrated. The 2l25-cycle tone current, which may come either from the inverter I2 or from the amplifier I3 or from both, is passed through the filter I6 and then rectified by the rectifier I8. The rectified current is impressed upon the differential relay I9 to operate the relay in the proper direction to cause a marking signal to be made in the local circuit or line section 20. In the case of a spacing signal, f is 2295'cycles as illustrated. The 2295-cycle current from inverter I2, amplifier I3, or both, is passed through filter I5 and rectified by rectifier I'l'. The rectified current acts upon the relay I9 to cause a spacing signal to be made in local circuit or line 20.

In the event of selective fading of the radio waves in traversing the space between antennae 9 and I0, the waves of one or more of the side frequencies F-l-, F-f, F|3000-f, or F-3000+f may be attenuated, or even practically eliminated before reaching the radio receiver II. Especially in case of single side-band transmission, where F-l-f and F-l-BOOO-f are the transmitted frequencies assuming that it is the upper sideband that is transmitted, one of the frequencies f or 3000-f may at times beabsent from the output of receiver I I. Suppose, for example, that f is absent. Provided 3000-f is present, the current of this frequency causes a current of frequency f to be produced in the inverter I2. This inverted current will pass filter I6, assuming a marking signal, and will actuate the relay If at another time, the frequency f alone is present, the current of this frequency passing through the amplifier I3 will cause relay I9 to be actuated as before. Thus positive action of the relay I9 is secured regardless of failure of one of the tone currents. The probability of simultaneous fading, which would eliminate frequencies f and 3000-f together, has been found by experience to be much less than the probability of selective fading of the kind assumed above wherein if either tone frequency is eliminated temporarily the other remains. Therefore an effect of the invention is to reduce the chances of loss of signals in the presence of fading.

Fig. 2 shows an alternative arrangement of the transmitting circuits. As indicated, the key 3 has two back contacts which are connected respectively to the tone generator I, producing the 2295-cycle current and to another tone generator 2|, producing a 70S-cycle current. The key also has two main contacts which are connected respectively to the generator 2, of 2125 cycles, and to a generator 22 producing current of 875 cycles. The key is connected through the line 4 directly to the transmitter 8. At the receiving station the circuits may be as shown in the receiving portion of Fig. 1.

In the operation of the system of Fig. 2, there is no inverter required at the transmitting station, for the frequencies f and 3000- f are supplied by individual generators. A marking signal is sent, as before, by depressing the key. Now the main contacts of the key connect generators 2 and 22 jointly to line 4, the generator 2 supplying frequency f and the generator 22 supplying frequency 3000-f, where f is the marking frequency. A spacing signal is sent, as before, by releasing the key, this time connecting generators I and 2| to the line 4, thus supplying the frequencies ,f and 3000-f, where f is the spacing frequency. From this point on the operation is the same as in the system of Fig. l.

Fig. 3 shows a modified system using in reception two modulators, either or both of which may be inverters. The transmitting station is arranged the same as the station shown in Fig. 2, except that the frequencies assigned to the oscillators are changed. At the receiving station, the general arrangement is like that shown in Fig. 1. A modulator 23 and attendant oscillator 24 are shown in place of the inverter I2 and oscillator I4. Another modulator 25 with attendant oscillator 26 is shown in place of the amplifier I3. 'I'he filters I5 and I6 have had their tuning changed to 1000 cycles and 800 cycles respectively. The rectifiers IT, I8 are shown connected to individual relays or other responsive devices 21 and 28 respectively.

In the operation of the system of Fig. 3, a marking signal is sent by means of 2900 cycles from oscillator 2 and 600 cycles from oscillator 22. Upon receipt of a Z900-cycle current at modulator 23 the sum and difference frequencies 6600 cycles and 800 cycles are produced and irnpressed upon the filters I5 and I 6, tuned to 1000 cycles and 800 cycles respectively. The 800-cycle current is transmitted by lter I6, and rectified at I 8 to furnish current for operation of relay 28. The 6600-cycle current is readily rejected or attenuated by both lters. The 2900-cycle current is also incident upon the modulator 25 which produces therefrom currents of 4300 cycles and 1500 cycles respectively, which fail to pass either filter. Upon receipt of a 600-cycle current the modulator 23 produces currents of 4300 cycles and 3100 cycles respectively, which are rejected by both filters. The 600-cycle current incident upon the modulator 25 produces currents of 2000 cycles and 800 cycles respectively, of which the 800-cycle current passes through the filter I6 and is rectified, either causing or aiding relay 28 to operate. Thus either 2900-cycle current or 600-cycle current in the output circuit of the radio receiver II will serve to cause a signal to be received by relay 28. In either case the detected tone current, 2900 cycles or 600 cycles, is converted to a common value, namely 800 cycles, the frequency to which filter I6 is responsive.

A spacing signal is sent by means of Z700-cycle current from oscillator I and 40G-cycle current from oscillator 2l. Receipt of a Z700-cycle current by modulator 23 causes currents of 6400 cycles and 1000 cycles, respectively, to be produced, the 1000 cycles being then transmitted through filter I and used to operate the relay 21 in the same manner as current passed by filter IG serves to operate the relay 28. The Z700-cycle current incident upon the modulator 25 produces Lt100-cycle and I300-cycle currents which are rejected by both filters. Receipt of a 400-cycle current by modulator 23 causes the production of 4l00-cycle and 3300-cycle currents which are discarded, while the 40G-cycle current produces 1800-cycle and 1000-cycle currents in modulator 25. The 1000-cycle current thus produced is transmitted through filter I5 to cause operation of relay 21. Therefore, either 2700-cycle current or 40G-cycle current in the output of the radio receiver will be brought by modulation to the common value 1000 cycles and will be effective to cause a response in relay 21.

It will be observed that in the system as illustrated, the useful currents generated by the modulators 23, 25 are lower side frequencies in each case. In producing a lower side frequency the function of the modulator is the same as that of an inverter. It is not necessary, however, to employ only lower side frequencies. In other words, the modulators 23 and 25 need not be inverters, although one or both may be inverters similar to 5 and I2.

It will be evident that the particular frequencies assigned to the tone generators I, 2, 2| and 22. and to the carrier generators 1, I4, 24 and 26, as illustrated herein, need not be used and others may be substituted as desired. It is, however, desirable to use substantially the same frequency in generator I4 as in generator 1, in order that a frequency inverted by inverter 5 may be reinverted to its original value by inverter I2. It is only essential that the two frequencies used in a marking signal be shifted to the same frequency value before reaching the filter I5 and that the two frequencies used in a spacing signal be brought to a different common value before entering the filter I6. The common value is, of course, the frequency to which the respective filter is tuned. By means of the system of Fig. it is illustrated how the filters I5 and I6 may be tuned to any arbitrary frequency desired, provided two modulators or inverters are used instead of one as in Fig. 1. When the frequencies of the filters have been selected, the frequencies assigned to the oscillators I, 2, 2|, 22, 24 and 26 must be given a set of frequency values consistent with the choice of the filter frequencies. In such selection of frequencies, it is desirable to avoid having any frequency which the modulators can produce from a marking signal lie in or close to the filter assigned for the reception of spacing signals, and vice versa. In the system of Fig. 3 as illustrated, it will be noted that the closest approach to 1nterference between marking and spacing signals occurs on account of a 1300-cycle current produced by modulator 25 during a spacing signal, the 1300 cycles being but 300 cycles away from the resonant frequency of the filter I5. The frequency of filter I5 is 1000 cycles and the lter is intended to respond exclusively to markinglsignals.

It will be evident that the amplifiers 6 and I3 may not always be needed to bring the currents transmitted therethrough to a suitable energy level. It is desirable, however, in any case to protect the system from feedback effects which arise if the path through the amplifier is not restricted to unidirectional transmission in the forward direction. The usual unidirectional type of amplifier provides a convenient means of preventing feedback and at the same time the gain of the amplifier may be adjusted in known manner to regulate the amount of current impressed upon the succeeding apparatus.

The invention is notrestricted to radio transmission but may be used for direct wire communication, with or without the use of a carrier wave, or in a system including any kind of communicating link in which a single frequency wave may be subject to uncontrolled attentuation or fading.

The key 3 may be replaced by a telephone dial switch or by any manually or automatically operated switch or impulse forming device. Also the terms key and keying are to be understood as including dial switches and the operation of dialing, respectively.

It will be evident that in the system of Fig. l, the filters I5 and I6 need not be tuned to the frequencies designated by f above but may equally well be tuned to the frequencies designated by BDOO-f.

In the system of Fig. l, the relay I9 need not be a differential relay but may be a relay of any desired type, or it may be a telegraph sounder, a local indicator of signals or any suitable electrically operated device. When a local device is to be operated the line 20 may not be needed. In case the relay I9 is operable by alterhating current of the frequencies passed by the filters I5, I6, th-e rectifiers I1, I8 may be dispensed with. It should be noted that the use of the differential relay has a certain advantage in the presence of static or other extraneous disturbing effects, because any currents or transients of a nature to affect the filters I5, I6 equally well may be substantially balanced out so as to have no resultant effect upon the relay I9.

The terms marking signa and spacing signal as used are to be understood as illustrative of any two kinds of signals which it may be desirable to employ. It is evident that the system may be used to send marking signals alone with spaces between which will serve in the usual way as spacing signals. It will also be evident that the invention is not limited to the use of two frequencies for sending a signal. Three or more frequencies may be employed simultaneously in order further to increase the probability that one frequency will be transmitted if all the others fade out. An additional modulator may be provided for each added frequency and the attendant carriers may be so selected that all the frequencies used to carry the same signal are translated to a common frequency value before being used to operate the final receiving device.

What is claimed is:

1. A system comprising means for keying a tone frequency wave to develop code signals, a source of carrier Waves, means actuated by said source to translate the frequency of said keyed tone wave through a frequency interval xed by the carrier frequency, means to transmit both the translated and the untranslated tone wave, means at a receiving station to translate one of said keyed Waves by the said fixed frequency interval, whereby the received Waves are brought to a common frequency, selective means tuned to said common frequency, and a detector for said selected wave.

2. A system comprising means for keying a tone frequency wave to develop code signals, means for inverting the frequency of said tone with respect to a substantially fixed carrier frequency, means to transmit both the inverted and the original tone wave, a pair of circuits at a receiving station, means in one of said receiving circuits for inverting the frequency of an impressed wave with respect to substantially the same carrier frequency above mentioned, means in the other of said receiving circuits to by-pass said inverting means, a filter tuned to said original tone frequency, said filter being connected both to the output of said last-mentioned inverting means and to said by-passing circuit, and means for detecting signals carried by waves passing through said lter.

3. A system comprising a source of tone frequency current, keying means for forming current from said source into pulses in accordance with a code, a divided circuit connected to the keying means and containing in one branch a unidirectional amplifier and in the other branch av frequency inverting means, a transmission system connecting the combined output circuits of the amplifier and of the inverting means to a second divided circuit, said second circuit having component parts substantially identical with those in said first-mentioned divided circuit, a filter connected to the output terminals of said second divided circuit, and means in the output circuit of the filter for detecting code-formed current pulses.

4. In a telegraph system, a tone generator of a predetermined frequency for marking signals, a tone generator of another frequency for spacing signals, a transmission system subject to selective fading, keying means for connecting said generators alternately to the transmission system, frequency inverting means inserted between the keying means and the transmission system whereby there is caused to be superposed upon the current transmitted from whichever generator is connected through said keying means an auxiliary current, of a frequency other than that of said connected generator, tuned means at the far end of the transmission system for separately detecting marking signals and spacing signals, and frequency.inverting means inserted between the transmission system and the detector for superposing upo nthe auxiliary current a current of the originally transmitted frequency whereby suitable signaling current is locally supplied to the detecting means in case of fading of the main current in the transmission system.

EDWARD VROOM. 

